Coronal Holes: The (Rarely Round) Gaps In The Sun's AtmosphereScientists aren't sure exactly why holes form in the hot and glowing outermost layer of gas surrounding the sun. But one theory is that the dark blotches we see on images of the sun could be the remnants of the (relatively) cool splotches called sunspots.

NASA's Solar Dynamics Observatory captured this picture of the sun on June 18. The dark blue area in the upper left quadrant of the sun is a huge coronal hole more than 400,000 miles across. Coronal holes are areas of the sun's outermost atmospheric layer — the corona — where the magnetic field opens up and solar material quickly flows out.
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NASA's Solar Dynamics Observatory captured this picture of the sun on June 18. The dark blue area in the upper left quadrant of the sun is a huge coronal hole more than 400,000 miles across. Coronal holes are areas of the sun's outermost atmospheric layer — the corona — where the magnetic field opens up and solar material quickly flows out.

NASA/SDO

There's a hole in the sun's corona. But don't worry — that happens from time to time.

"A coronal hole is just a big, dark blotch that we see on the sun in our images," says Dean Pesnell, project scientist for NASA's Solar Dynamics Observatory. "We can only see them from space, because when we look at them [through] a regular telescope, they don't appear."

That's because you have to look at wavelengths of light that the human eye can't see. As the name suggests, coronal holes are holes in the sun's corona, not the sun itself. The corona is a hot and glowing outer layer of atmosphere that surrounds the sun. It extends millions of miles into space.

Pesnell says scientists aren't really sure where coronal holes come from. "Some people claim that they are the skeleton of old sunspots," he says.

Sunspots are also dark blotches, but they're on the surface of the sun, below the corona. They're caused by strong magnetic fields that cause cool regions — which look dark — to form on the sun's surface. The idea is that when sunspots fade away, they leave behind a coronal hole.

One thing scientists do know for sure about coronal holes is that they're not round. "They have all kinds of cool shapes," Pesnell says. "We've seen [some] that look like rubber chickens. My favorite is the one that looks like Kokopelli, a flute player from Native American cultures in the Southwestern U.S."

Different Views Of The Sun

NASA's Solar Dynamics Observatory launched in 2010 with the goal of understanding the sun's influence on Earth and "near-Earth space." The probe has a suite of different imaging tools, which capture images of the sun in different wavelengths of light, or "channels." The images below were all captured by the SDO on Aug. 23.

This image highlights the outer atmosphere of the sun, called the corona, as well as hot flare plasma. Hot active regions, solar flares and coronal mass ejections appear bright here. The dark areas, called coronal holes, are places where very little radiation is emitted. But these holes are the main source of solar wind particles.

Solar Dynamics Observatory/NASA

This channel is especially good at showing areas where cooler, dense plumes of plasma above the visible surface of the sun are located. Many of these features either can't be seen or appear as dark lines in the other channels. The bright areas show places where the plasma has a high density.

Solar Dynamics Observatory/NASA

This channel is particularly good at showing coronal loops — the arcs extending off the sun where plasma moves along magnetic field lines. The brightest spots seen here are places where the magnetic field near the surface is exceptionally strong.

This channel is designed to study solar flares. It measures extremely hot temperatures around 18 million degrees Fahrenheit, as well as cool plasmas around 720,000 degrees Fahrenheit.

Solar Dynamics Observatory/NASA

This channel highlights the active region of the outer atmosphere of the sun. Active regions, solar flares, and coronal mass ejections will appear bright here. The dark areas, or coronal holes, are places where very little radiation is emitted; they are also the main source of solar wind particles.

Solar Dynamics Observatory/NASA

This channel is designed to study solar flares. It measures extremely hot temperatures around 10.8 million degrees Fahrenheit.

Solar Dynamics Observatory/NASA

This channel often shows a weblike pattern of bright areas that highlight places where bundles of magnetic field lines are concentrated. However, small areas with a lot of field lines will appear black, usually near sunspots and active regions.

Solar Dynamics Observatory/NASA

This channel (like the previous one) often shows a weblike pattern of bright areas that highlight places where bundles of magnetic fields lines are concentrated.

Solar Dynamics Observatory/NASA

This channel shows the features that our eyes would see if we could dim the sun's intensely bright light. This wavelength of light is visible to people as blue-indigo, although here it is shown in yellow. Sunspots stand out sharply here, and you can also see that the edge of the sun appears darker, a well-known effect called limb darkening.

Solar Dynamics Observatory/NASA

This image combines three images with different, but very similar, temperatures. The colors are assigned differently than in the single images. Each highlights a different part of the corona.

Solar Dynamics Observatory/NASA

This image combines three images with quite different temperatures.

Solar Dynamics Observatory/NASA

This image combines three images with different temperatures.

Solar Dynamics Observatory/NASA

This image shows the magnetic field directions near the surface of the sun. White and black areas indicate opposite magnetic polarities, with white showing north (outward) polarity and black showing south (inward) polarity.